How much valve over lap with turbo?

[oil pan 4]

I have to build a new boostable 3.4 now.
I was gong to run up to 14 psig or more.
My last one (engine & cam) got burned up in a shop fire after it sould have been in my car and gone from that shop.
What cam sould I get? Want is the most over lap I can run with turbo? I had the crane 272 cam with one degree of over lap.
I want to go bigger if I can.
I need to get a new 3.4 and cam asap with money I was saving for a (big) plasma cutter.

[Turbo V6 Camaro]

why dont you go 3.8? the motor got burnt up or the car? both??

[oil pan 4]

just the engine.
I'm keeping the 3.4 because I have tons of mods all ready done for it.

[Turbo V6 Camaro]

cool well you going to turbo or SC ? the cams are vry different for turbo the SC

and the more lobe over lap you have to more of a "LUB" the cam makes

[bobrules]

Guy I know who's running a turbo (albeit not on a 6) is running 4* of negative overlap. If I'm thinking right, overlap will have both the intake and exhaust valves open at the same time for a certain period of time. This means that you put "x" amount of boost into the engine, and for a certain period of time, your exhaust valve is open with your intake, therefore your not going to keep that positive pressure, it's going to blow out the exhaust port. It's useful on NA motors that want to make horsepower at the higher RPMs since it helps the engine breath...but it also takes away low end power and driveability.

Thats why FI is a nice alternative to keeping displacement down yet getting good power. You don't have to run any overlap at low RPMs, which lets you keep low end power for driving around town, but at higher RPMs the boost makes up for the inefficiency. Thats the whole idea of VVT...less overlap at lower RPM, with more at the top end.

[TurboProject]

Overlap on a turbo engine is not good. Any street turbo is going to have more exhaust backpressure that boost pressure. The only way to not have it this way is to get a huge turbine housing but then the turbo won't spool until upper RPMs, which is fine if you're doing racing and sitting at 6-7k all day but for the street you're going to want it lower which will give backpressure. If you have a cam with overlap, the more overlap, the more exhaust gets pushed into the intake.

[Arctc Wolf]

This depends greatly on when the boost will come on, and how restrictive the turbine side is. A turbo meant to make max boost for the top 2000-2500 rpm can actually use a tighter lobe separation.

A turbo designed for the midrange of an engine, which is most balanced for street and race, will require less overlap. This is because at higher rpm, the turbine becomes a restriction in the exhaust, causing an increase in backpressure. This will cause exhaust to reverse and go back into the cylinders and contaminate the fresh air/fuel charge. This is called reversion. It hurts power, emissions, cause detonation, higher engine temps, premature wear, ect. To prevent reversion, less overlap is helpful. Less of the backpressure can go back into the cylinder.

For turbos that are slow spooling, top end killers, more overlap can be used. What I mean by top-end killers, is that it isn't spooled up till about 2000-2500 before redline, maybe less depending on design and manufacturer. Because the turbo is so huge, at lower rpm, it can't spin the wheel to produce any boost, meaning that it has very low backpressure. This will allow more overlap at higher rpms because of the low backpressure. Having more overlap will also allow for a lot more power to be made as well, since better breathing at top end creates a lot more hp, as sen with NA cams and overlap for top-end performers. I doubt that u'll wanna go with as much overlap as u would on an NA car, but something with just a little less overlap would be good, perhaps aa couple degrees less overlap would work. As I said, it depends on the turbo being used. Ball-bearing turbos will spool a lot quicker than older styles will, causing a little more room at the top end for power. Also, the new designs from Garrett also help spool increase and backpressure reduction due to better aerodynamics.

Hope this helped some.